FIG. 1 is an engine cycle diagram depicting basic operation of a conventional four cycle internal combustion spark ignition piston engine that operates under the Otto cycle. Otto engines are used in powered vehicles, such as automobiles, trucks, and off-road vehicles, as well as in power equipment, such as lawnmowers, construction equipment, generators, air compressors, and the like. Otto engines typically mix a combustible fuel with air that is ignited to produce power. While gasoline is the most common type of combustible fuel that is used in an Otto engine, other types of fuels, including ethanol, methanol, propane, methane, and the like, can also be used. A popular combustible fuel in use today in the United States is a mixture of ethanol and gasoline with the ratio of ethanol to gasoline varying anywhere from as little as a few percent ethanol to as much as 85% ethanol.
FIG. 1 depicts Otto engine operation for an exemplary engine having at least one cylinder with two valves (not shown) per cylinder and a reciprocable piston received in each cylinder defining a combustion chamber therein. At the beginning of cycle 1, the intake stroke, the piston is located at or near a top-dead-center (TDC) position. Typically, at least air alone or in combination with fuel is drawn through an open intake valve (not shown) into the combustion chamber due to movement of the piston away from TDC toward a bottom-dead-center (BDC) position. While fuel can be mixed with air before the mixture is drawn into the combustion chamber, such as where a carburetor or single-point fuel injection is used, the fuel can be directly injected into the chamber such as where multi-point fuel injection is employed.
Once the intake stroke is completed, the intake valve closes in preparation for compression of the air-fuel mixture in the combustion chamber during the compression stroke, cycle 2. During the compression stroke, the piston moves within the cylinder toward TDC compressing the air-fuel mixture within the combustion chamber due to piston movement decreasing the volume of the chamber.
After the air-fuel mixture is suitably compressed, the mixture is ignited, typically with a spark discharged by a spark plug, during the power stroke, cycle 3, such that combustion of the mixture produces combustion gases that rapidly expand in the chamber increasing the pressure within the chamber. This causes a corresponding force to be exerted against the piston, which ultimately displaces the piston back towards BDC. Piston displacement is translated by a connecting rod linking it to a crankshaft into rotary power engine output.
To discharge the combustion gases after completion of the power stroke, an exhaust valve is opened during the exhaust stroke, cycle 4, enabling the gases to be expelled out an exhaust, such as an exhaust manifold that typically communicates with a muffler. After the exhaust stroke is finished, the exhaust valve closes. Thereafter, these four cycles repeat themselves as needed for continuous engine operation.
FIG. 2 illustrates exemplary pressure-volume plots of such an Otto engine showing a work plot of work inputted during the intake and compression strokes, and a power plot depicting net power outputted during the power stroke. Each plot assumes an ideal thermodynamic cycle, an 8:1 compression ratio, a specific heat ratio of approximately 1.4 for air, and combustion gases are exhausted during the exhaust stroke until a pressure of about 3.5 atmospheres when the exhaust valve is opened. The area under the dashed or phantom line in the work plot represents the engine power outputted during the power stroke shown in the power plot and the area between the abscissa and the bottom curve of the power plot represents the inputted work.
While theoretical maximum efficiency for an Otto engine represented by the plots in FIG. 2 is about 56%, in reality efficiency is far less. For example, it is not unusual for actual efficiencies to be less than half of theoretical in many Otto engine applications with typical maximum efficiency for an automobile engine being around 20-25%. Utility engines having a horsepower range of between 10 hp and 40 hp are usually even less efficient because they are often run rich to ensure consistent operation under a wide range of operating conditions.
While the Otto gasoline engine is the most popular engine in commercial use today, it is not without drawbacks and disadvantages. Most Otto engines cannot use more than one fuel without installation of expensive and sophisticated sensor systems that typically also require multi-point fuel injection to precisely meter fuel flow to accurately control air-fuel ratio. Similarly, almost all Otto engines require an expensive catalytic converter system to significantly reduce exhaust emissions. Additionally, Otto engines often operate at partial throttle where efficiency is even lower, often as low as about 10%.
These drawbacks and disadvantages are particularly true for utility engines that operate under the Otto cycle. These smaller engines typically have undesirably high exhaust emissions, typically in the range of 6-10 grams of hydrocarbons and nitrous oxides per horsepower hour, because it is not been presently found economical to equip them with catalytic converters. Because it is usually also not economical to equip such small engines with sophisticated mass flow sensors, engine control computers, fuel injection systems, gas recirculation systems, and the like, carbon monoxide emissions are usually also undesirably high because of the need to run rich to ensure consistent engine operation over a wide range of operating conditions.
Because of the need to keep utility engine costs economical, configuring these smaller utility engines to run rich to ensure consistent operation undesirably increases fuel consumption, which can range from 0.6 pounds per horsepower hour for wide open throttle up to as much as 1.3 pounds per horsepower hour at partial throttle. This also can cause combustion ignition and detonation problems with some engines also experiencing “after-bang” resulting from unburned fuel detonating when discharged from the engine during the exhaust stroke. Finally, such engines are usually loud, both during starting and during operation.
A Diesel engine operates somewhat similarly to an Otto engine except that it is a compression ignition engine where combustion in a Diesel engine takes place at constant volume rather than at constant pressure, which is possible with an Otto engine because it is a spark ignition engine. During the compression stroke of a Diesel engine, air in the combustion chamber is heated to a temperature high enough to ignite fuel injected into the combustion chamber without requiring any spark to incite ignition. While Diesel engines suffer from many of the same drawbacks and disadvantages as Otto engines, they also possess some unique drawbacks and disadvantages.
For example, while Diesel engines can use alternative fuels, fuel quality is especially critical because there is far less time to achieve vaporization and mixing with the compressed air to achieve compression ignition than there is for an Otto engine. Fuel must be injected right before the piston reaches the TDC position to ensure compression is great enough to achieve fuel ignition temperatures. If fuel quality is poor, such as if its Cetane rating is below 40, if it is not volatile enough, or if it has too high of viscosity, poor, no or incomplete combustion can result.
In addition, since fuel must be discharged into the combustion chamber at just the right time shortly before the piston reaches the TDC position to ensure the compressed air is hot enough to achieve compression ignition, more expensive fuel injectors and fuel injection control systems are required. Compressing air so it becomes hot enough to achieve compression ignition requires operation at a typical compression ratio of at least 14:1, which requires Diesel engines to be more strongly and heavily built. As a result, Diesel engines tend to cost significantly more such that very few utility engines are Diesel engines.
Another type of combustion engine most commonly associated with gas turbine engines is a Brayton engine that operates under the Brayton or Joule cycle. A Brayton cycle gas turbine engine typically includes a gas compressor, a burner or combustion chamber, and an expansion turbine where extracted work is outputted as power. Industrial gas turbines and jet engines are examples of such Brayton cycle engines.
However, before the Brayton cycle became so firmly associated with gas turbine engines, Brayton engines initially utilized a first reciprocating piston-cylinder arrangement as a compressor to compress air, a mixing chamber where fuel was mixed with compressed air where combustion of the air-fuel mixture took place, and another larger reciprocating piston-cylinder arrangement where expanded combustion gases acting on the piston provided power output. Some of the outputted power was inputted back into the engine as work to drive the compressor. Examples of Brayton-cycle piston-type combustion engines are disclosed in U.S. Pat. Nos. 5,894,729; 4,369,623; and 4,333,424. One other type of Brayton cycle piston-cylinder type engine is an Ericsson hot air engine, developed in the mid-1800's, which improved upon the original Brayton engine by including a recuperator or regenerator between the compressor and the expander that can increase engine efficiency.
While Brayton cycle gas turbine engines have enjoyed great commercial success, the Brayton cycle dual piston-cylinder engine counterpart to date has not. While a Brayton cycle dual piston-cylinder engine offers certain advantages over Otto and Diesel engines, significant hurdles have remained to date impeding their commercialization and acceptance. Therefore, improvements are desired that will facilitate commercialization and adoption of a Brayton cycle piston-cylinder type engine.